1. Field of the Invention
The present invention relates to a nonaqueous electrolyte battery, a battery pack using the nonaqueous electrolyte batteries, and a vehicle having the battery pack mounted thereto.
2. Description of the Related Art
A nonaqueous electrolyte battery comprising a negative electrode containing a lithium metal, a lithium alloy, a lithium compound or a carbonaceous material attracts attentions as a battery having a high energy density, and a vigorous research is being conducted on the particular nonaqueous electrolyte battery. A lithium ion battery comprising a positive electrode containing LiCoO2 or LiMn2O4 as an active material and a negative electrode containing a carbonaceous material capable of absorbing-releasing lithium has been widely put to the practical use. Also, it is studied to use a metal oxide or an alloy in place of the carbonaceous material in the negative electrode.
In general, a copper foil was used as the current collector of the negative electrode, and nickel or copper was used as it was as the terminal. For example, Japanese Patent Disclosure (Kokai) No. 2001-84993 is directed to a battery in which the external lead terminal of the positive electrode is formed of aluminum or an aluminum alloy. In the battery disclosed in this patent document, a porous oxide film is formed by a chemical treatment on the entire circumferential surface that is sealed by a film forming a case at the external lead terminal of the positive electrode. Also, according to this patent document, the negative electrode current collector is formed of copper or nickel. And in order to avoid the corrosion caused by the electrolysis solution, the external lead terminal of the negative electrode is formed of nickel.
However, if the battery comprising the negative electrode including a copper foil used as a current collector is put under an over discharge state, the potential of the negative electrode is elevated to promote the dissolving reaction of the copper foil, with the result that the discharge capacity is rapidly lowered. Such being the situation, it is necessary for the battery to include a protective circuit that is effective for preventing the battery from being put under an over discharge state. However, it is disadvantageous in view of the energy density to mount the protective circuit.
Under the circumstances, it is proposed in, for example, Japanese Patent Disclosure (Kokai) No. 2002-42889, to use an aluminum foil or an aluminum alloy foil as the negative electrode current collector, and a specified negative electrode active material. The use of the aluminum foil or the aluminum alloy foil makes it possible to provide a nonaqueous electrolyte battery exhibiting an improved energy density and an improved charge-discharge cycle performance.
On the other hand, a nonaqueous electrolyte battery using an aluminum foil or an aluminum alloy foil as a negative electrode current collector is considered hopeful as a battery mounted to a vehicle such as an assistant bicycle, an electric scooter, an electric automobile or a hybrid automobile or as an emergency power source of an electronic apparatus. The battery mounted to a vehicle is required to be charged rapidly when the battery is put under an environment of a high temperature and a high humidity for a long time, e.g., the temperature not lower than 60° C. and the relative humidity not lower than 90%, and also required to achieve the discharge with a high output.
If the case in which an electrode group is to be housed is formed of a film in order to decrease the weight of the nonaqueous electrolyte battery, the hermetic sealing performance of the case is lowered if the battery is used for a long time under a high temperature environment. As a result, the water permeates into the case to bring about problems that the discharge capacity is lowered and that the charge-discharge cycle performance is lowered.